The present invention relates to an injection moldable electrically conductive composition comprising aromatic thermoplastic liquid crystalline polymers (LCPs), electrically conductive articles made therefrom, and the injection molding process for making them. The compositions of the invention are useful in a wide variety of applications including electrochemical devices such as battery current collectors, high efficiency electromagnetic/radio frequency interference shielding, and electrostatic dissipative packaging and equipment housings. The present invention is particularly useful in fuel cells.
In the current state of the art, a typical fuel cell comprises the elements shown in FIG. 1. A membrane/electrode assembly (MEA), 10, comprising a membrane separator, 11, and catalyst coatings, 12, on either side thereof, and two (2) gas difflusion backing sheets, 20, are sealed by gaskets, 30, between two (2) electronically conductive graphite plates, 40. The plates often serve a multiple role as current collectors conveying electrons to the external load via electrical connections not shown, as mechanical supports for the other fuel cell components, and as gas and water distribution networks via a pattern of flow fields inscribed upon the surfaces thereof, 50. Gas and water inputs and outputs are generally integral with the graphite plate, but are not shown. The graphite plates normally serve as the interface between adjacent cells in a stack. The plates are known variously as current collectors, flow fields, and bipolar (or monopolar) plates. For further information, see, for example, Ullmann""s Encyclopedia of Industrial Chemistry, 5th ed. Vol. 12A, pp. 55ff, VCH, New York, 1989.
Because of its multiple role, the bipolar plate has a number of requirements to meet. The plate must have good electrical conductivity, good mechanical or structural properties and high chemical stability in the chemically reactive fuel cell environment. In addition because of its gas distribution role it must be made of a gas impermeable material and be formed with complex gas delivery channels across its surface.
In the current practice of the art, graphite is the material of choice for bipolar plates because of its high electrical conductivity, high strength and immunity to corrosion. However, it is brittle, expensive, and requires expensive machining to produce. The brittleness of graphite necessitates its use in ca. six (6) mm thick slabs which adds both weight and bulk to the fuel cell, thus driving down its power density (kW/l or kW/kg) in use.
Carbon/graphite filled thermoplastic polymers have long been identified as a promising alternative to graphite in bipolar plates. In principle, conductive, reinforced thermoplastic polymer compositions can be molded directly into complex, intricate shaped components using low cost, high-speed molding processes. In addition, these more ductile materials will enable the development of new stack designs because moldable plastics offer much greater flexibility to the form of fuel cell components. Unfortunately, this potential has not been realized in the art despite numerous attempts to do so.
Electrically conductive thermoplastic polymer compositions providing volume resistivities of 10xe2x88x923-10xe2x88x922 ohm-cm are known in the art, and are of particular interest in the production of fuel cell current collectors.
U.S. Pat. No. 3,945,844 to Nickols discloses polymer/metal composites. Polysulfone, polyphenylene sulfide, polyphenylene oxide, acrylonitrilebutadiene-styrene copolymer are combined in a variety of ways with stainless steel, silver, gold, and nickel. The amount of either metal powders or fillers or both, in the polymer/metal composite varies from 50 to 80 weight % percent. Resistivity levels as low as 10xe2x88x923 ohm-cm are reported.
U.S. Pat. No. 4,098,967 to Biddick et al. provides a bipolar plate formed of thermoplastic resin filled with 40-80% by volume finely divided vitreous carbon. Plastics employed in the compositions include polyvinylidene fluoride and polyphenylene oxide. The plates are formed by compression molding dryblended compositions and possess specific resistance on the order of 0.002 ohm-cm. Compression molded bipolar plates from solution blends of graphite powder and polyvinylidene fluoride are disclosed in U.S. Pat. No. 3,801,374 to Dews et al. The plate so formed has a density of 2.0 g/cc and volume resistivity of 4xc3x9710xe2x88x923 ohm-cm.
U.S. Pat. No. 4,214,969 to Lawrance discloses a bipolar plate fabricated by pressure molding a dry mixture carbon or graphite particles and a fluoropolymer resin. The carbon or graphite are present in a weight ratio to the polymer of between 2.5:1 and 16:1. For polymer concentrations in the range of 6-28% by weight, volume resistivity is in the range of 1.2-3.5xc3x9710xe2x88x923 ohm-in.
In U.S. Pat. No. 4,339,322 to Balko et al., the physical strength of the compression molded composite of U.S. Pat. No. 4,214,969 was improved by substituting carbon fibers or other fibrous carbon structures for some of the graphite powder. Typical composition includes 20% (by weight) polyvinylidene fluoride (PVDF), 16% (by weight) carbon fiber, and graphite powder. The dry mixture was blended, then pressure molded into plates. The volume resistivity is in the range of 1.9xc3x9710xe2x88x923 to 3.9 10xe2x88x923 ohm-in at a binder/resin loading levels of 7-26 wt %.
U.S. Pat. No. 4,554,063-85 to Braun et al. discloses a process for fabricating cathode current collectors. The current collector consists of graphite (synthetic) powder of high purity, having particle sizes in the range from 10 (micron) to 200 (micron) and carbon fibers which are irregularly distributed therein and have lengths from 1 mm to 30 mm, the graphite powder/carbon fiber mass ratio being in the range from 10:1 to 30:1. The binder/resin used is polyvinylidene fluoride. For producing the current collector, the binder is dissolved in, for example, dimethylformamide. Graphite powder and carbon fibers are then added and the resulting lubricating grease-like mass is brought to the desired thickness by spreading on a glass plate and is dried for about 1 hour at about 50xc2x0 C. The plates were also formed by casting, spreading, and extrusion.
U.S. Pat. No. 5,582,622 to Lafollette discloses bipolar plates comprising a composite of long carbon fibers, a filler of carbon particles and a fluoroelastomer.
Also known in the art is the use of metal-coated, particularly nickel-coated chopped graphite fibers to form conductive polymer compositions. In order to reduce fiber attrition by compounding, the prior art discloses employing a thermoplastic resin-impregnated bundle of nickel-coated graphite fibers which are directly injection moldable with a thermoplastic matrix resin with only a preliminary dry-blending step. See for example Kiesche, xe2x80x9cConductive Composites Find Their Niche,xe2x80x9d Plastics Technology, November 1985, P. 77ff; Murthy et al, xe2x80x9cMetal Coated Graphite Fiber Structural Foam Composites,xe2x80x9d Fourteenth Annual Structural Foam Conference and Parts Competition, The Society of the Plastics Industry, Inc., April 1986, PP 86ff. Use of wider gates and flow channels in molding machines processing graphite fibers is disclosed for example in International Encyclopedia of Composites, S. Lee, ed. pp 474ff, VCH publishers, 1990. Also disclosed therein is the enhancement of conductivity realized by orientation of high aspect ratio conductive fibers in the polymer matrix during the molding process.
Methods for forming resin-impregnated graphite fibers which are also applicable to metal-coated graphite are known in the art. Some of these methods are disclosed in xe2x80x9cGraphite Fiber Composites (Electrochemical Processing)xe2x80x9d by J. Iroh in Polymeric Materials Encyclopedia, J. C. Salamone, ed., pp. 2861ff, CRC Press 1996.
The art hereinabove cited is directed to replacing pure metal or graphite components which require extensive machining to be formed into finished articles with moldable compositions based upon thermoplastic polymer resins which require less, post-molding machining to form the finished article.
The problem in realizing the advantages of molded thermoplastic polymer parts has been related to the inverse relationship between concentration of conductive filler on the one hand and processibility and mechanical properties on the other. In practice, as shown in the art hereinabove cited, quantities of conductive filler required to achieve the 10xe2x88x922 ohm-cm resistivity goal in fuel cells result in products with limited practical utility. This is particularly true in regard to the formation of current collectors in fuel cell applications.
It is desirable to achieve a combination of properties and processibility in an injection moldable composition without the limitation on practical utility. Another advantage desired is the reduction in cost of forming finished articles such as current collectors in comparison to conventional methods.
Briefly stated, and in accordance with one aspect of the present invention, there is provided a process for fabricating a shaped article having a volume resistivity of less than 10xe2x88x922 ohm-cm, the process comprising:
combining an injection moldable aromatic thermoplastic liquid crystalline polymer resin and a composition comprising nickel-coated graphite fibers impregnated with a non-liquid-crystalline thermoplastic binder resin, to form a mixture at a temperature below the melting point of the thermoplastic liquid crystalline polymer resin, the graphite fibers being of a length of less than 2 cm and comprising about 5 to about 50% by weight of the mixture, and the binder resin comprising about 0.1 to about 20% by weight of the graphite;
feeding the mixture to an injection molding machine wherein the thermoplastic liquid crystalline polymer resin is melted and fed in the molten state to a mold; cooling the mold to a temperature at which the thermoplastic liquid crystalline polymer in the mixture no longer flows; and,removing the molded mixture from the mold.
Pursuant to another aspect of the invention, there is provided a shaped article having a volume resistivity of less than 10xe2x88x922 ohm-cm comprising about 50 to about 95% by weight of an aromatic liquid crystalline polymer about 5 to about 50% by weight of a nickel-coated graphite fiber of a length less than 2 cm, and about 0.1 to about 20% by weight with respect to the weight of the graphite fiber of a non-liquid-crystalline thermoplastic binder resin.
Pursuant to another aspect of the invention, there is provided a process for fabricating an electrically conductive shaped article, the process comprising: combining an injection moldable aromatic thermoplastic liquid crystalline polymer resin in the form of particles characterized by a mean particle size of less than 1500 micrometers with a graphite filler to form a mixture at a temperature below the melting point of the thermoplastic liquid crystalline polymer resin, the graphite filler being present in a concentration of about 5% to about 80% by weight of the total mixture; feeding the mixture to an injection molding machine wherein the aromatic thermoplastic liquid crystalline polymer resin is melted and fed in the molten state to a mold; cooling the mold to a temperature at which the resin in the mixture no longer flows; and, removing said molded mixture from the mold.